This invention relates to method and apparatus for atomizing and burning liquid fuel, and more particularly for forming a fuel/air mixture for a combustion chamber of a gas turbine installation.
In addition to a large number of parameters which determine the efficiency of a gas turbine and which affect both the design layout of all the individual components of a gas turbine and its mode of operation, the atomization process, by means of which the liquid fuel is to be atomized to form a fuel/air mixture which is as homogeneous as possible, plays a very decisive role in the fuel firing. In order to make it possible to carry out the combustion of the liquid fuel as completely as possible, it is the task of the fuel nozzles to atomize the liquid fuel into the finest possible fuel droplets in order to achieve, in this way, the largest possible fuel surface.
The simplest and lowest-cost fuel atomizers for liquid fuel are represented by pressurized fuel atomizers by means of which the fuel is driven through a nozzle opening at high pressure. Such, so-called SIMPLEX, atomizer nozzles are employed in combustion chamber operating concepts with burner staging and are suitable for the complete power range of a gas turbine, i.e. from the ignition process to the point where basic load operation is achieved. However, the employment of burner staging is very greatly limited because of the severe requirements with respect to the ignition process and with respect to the average temperature difference factor (OTDF) in the region of the turbine inlet. Thus the following relationship applies for the temperature difference factor OTDF:   OTDF  ⁢      xe2x80x83    ⁢                    T        MAX            ⁢                        T          _                H                                      T          _                H            -              T        C            
with
TMAX Maximum temperature at the turbine inlet
{overscore (T)}H Average temperature at the turbine inlet
TC Air temperature at the combustion chamber inlet (before combustion)
As a consequence of this, single-stage atomizers are exclusively employed in so-called silo combustion chambers in which one burner stage is provided, whereas multistage atomizer units, such as air-supported and compressed-air-supported atomizers are frequently employed in annular combustion chambers.
The fundamental problem in the design and layout of liquid fuel atomizer units is the quite different fuel flow rates at which the atomizer units are supplied during the operation of a gas turbine installation, starting with the ignition event and extending to the achievement of basic load operation. Thus, fuel flow rates under typical ignition conditions are less by a factor of between 10 and 20 than those under base load conditions. Also associated with this is the fact that the pressure ratios within the gas turbine installation are subjected to large changes, changing lay up to more than a factor of 100. Thus, typical pressure values for the atomization of liquid fuel under base load conditions are approximately 60 bar, whereas the atomization pressure under ignition conditions drops to between 300 and 600 m/bar. Pressure conditions are therefore reached which make it impossible to employ atomization nozzles designed for operation under base load conditions.
The invention is based on the object of providing a method and an appliance for atomizing liquid fuel for a firing installation, preferably for a combustion chamber of a gas turbine installation, having a nozzle arrangement through which the pressurized liquid fuel passes and is atomized to form a fuel/air mixture, in such a way that despite the large pressure differences described above, a single atomization unit is sufficient for undertaking the atomization necessary for optimized combustion of liquid fuel. This arrangement is to dispense with multiple staging, known per se, of the atomizing units. In particular, the atomizer appliance necessary for this purpose is to be of simple construction and be associated with only low manufacturing costs. It shall be possible to match the atomization rate and the achievable fuel droplet diameters in an optimum manner for both the ignition process and base load operation.
The invention derives from the basic concept that the minimum droplet size which can be achieved during atomization of a fluid by means of a pressurized atomizer unit is determined by the equilibrium between the surfaces tension, which holds a droplet together in its spherical shape, and the aerodynamic forces acting on the droplet from the outside, which aerodynamic forces can destroy the shape of the droplet. Thus, in the case of large droplet diameters, the aerodynamic forces are dominant so that, after the atomization process, the large droplets are really torn apart and disintegrate into smaller droplets. This process of bursting asunder into smaller droplets takes place until the surface tension becomes sufficiently large relative to the aerodynamic forces for further disintegration into even smaller fuel droplets to be prevented. This disintegration process leads to a droplet diameter which can be described by the following relationship:                     D        =                  C          ⁢                                                    xe2x80x83                            ⁢                                                ρ                  LIQUID                                ⁢                γ                                                                    ρ                GAS                            ⁢                              u                R                2                                                                        (        1        )            
with
xcex3 Kinematic surface tension
PLIQUID Density of the atomized liquid
PGAS Density of the surrounding gas
U2R Relative velocity between droplets and surrounding gas
C Constant
It may be seen from the above relationship that the droplet diameter D varies as the reciprocal of the square of the relative velocity between the atomized droplets and the gas surrounding the droplets. If, on the other hand, the supply pressure necessary for the atomization process (with which, for example, the liquid fuel is supplied to the atomization nozzle) is limited, only small relative velocities u are achieved so that the reduction in droplet size is unsatisfactory in terms of the finest possible atomization. This applies particularly in gas turbines during their ignition phase, in which the supply pressure within the turbine is relatively low.
In order, nevertheless, to achieve satisfactory atomization of the liquid fuel under the pressure conditions which make the atomization process difficult, a method in accordance with the preamble to claim 1 is developed, in accordance with the invention, in such a way thatxe2x80x94after passage of the fuel through the atomization unit configured as a nozzle arrangementxe2x80x94at least two, spatially separated fuel sprays are formed in which the fuel is mainly present in the form of individual fuel droplets. The fuel droplets each have a relative propagation direction such that the fuel droplets of one fuel spray collide with the fuel droplets of the other fuel spray in such a way that, during this collision of the fuel droplets, new fuel droplets are formed whose diameter is smaller than that of the colliding fuel droplets.
In contrast to the widespread concept of natural droplet disintegration due to the interaction between the surface tensions and the aerodynamic forces acting on the individual droplets, the method in accordance with the invention makes use of deliberate collision between fuel droplets after their formation as part of the atomization process.
By means of deliberate collision between fuel droplets which are formed when using a simple nozzle arrangement and an atomization pressure of approximately 500 mbar, i.e. a pressure which is usual for ignition in a gas turbine (these droplets having typical droplet diameters in the range between 2 and 5 mm), it is possible to obtain very small droplets, which emerge as xe2x80x9cfragmentsxe2x80x9d from the colliding droplets and have diameters between 10 and 100 xcexcm. The downstream xe2x80x9catomization processxe2x80x9dxe2x80x94based on the collision processxe2x80x94into still smaller droplet fragments does not correspond to the above relationship (1) because the physical mechanism which contributes to the reduction in size of the droplets is not based on the interaction between the surface tension and the aerodynamic forces acting on the individual droplets but on the collision between two droplets which consist of the same medium, of a combustible liquid in the case of the atomization of fuel. The mathematical relationship (1) simplifies, in fact, to the following relationship:                     D        ≈                  γ                      u            R            2                                              (        2        )            
Because of the disappearance of the density factors in Equation (2), a minimum droplet diameter can be obtained which is smaller by between two and three orders of magnitude as compared with classical droplet formation in accordance with Equation (1). This information on the atomization of liquid can, in accordance with the invention, be applied particularly appropriately to fuel atomization for use in gas turbines, particularly in view of the only low pressure ratios such as occur during the ignition phase in gas turbines.
A particularly advantageous possibility for producing the smallest fuel droplets by means of collision is initially based on the formation of at least two fuel sprays which can be generated within the scope of conventional atomization techniques. The fuel sprays, whose individual fuel droplets typically have droplet diameters of an order of magnitude between 1 and 5 mm, are preferably in the shape of a two-dimensional spray and their propagation directions are set relative to one another in such a way that they intersect at an acute angle. In the region of the mutually penetrating fuel sprays, collisions occur between the respective fuel droplets and these lead to extremely small fuel droplet fragments which preferentially adopt a propagation direction which is oriented along the angular bisector between the propagation directions of the two-dimensional fuel sprays which have collided with one another.
The collision geometry is typically matched to the individual combustion chamber geometry of annular combustion chambers in such a way that the extremely fine fuel droplets proceed in the direction of the combustion chamber for subsequent ignition.
A nozzle arrangement in accordance with the invention, and which operates on the atomization principle previously described, provides for at least two spatially separated nozzle outlet openings which are oriented relative to one another in such a way that the fuel sprays propagating in respectively different directions pass through a region within which the fuel droplets from the respective fuel sprays collide with one another. The nozzle outlet openings are therefore oriented relative to one another in such a way that the propagation directions of the fuel sprays emerging from the nozzle outlet openings enclose an angle a, for which 0xc2x0 less than xcex1 less than 180xc2x0.
As an alternative to the arrangement of at least two separate nozzle outlet openings, a nozzle arrangement is provided, in accordance with the invention, having a slot nozzle which has an endless slot nozzle opening. In this arrangement, the slot nozzle opening is preferably surrounded by a deflection element which deflects the fuel emerging from the slot nozzle opening in such a way that the fuel spray forming converges within a narrowly limited volumetric region. Such a slot nozzle arrangement has, in particular, the advantage that no complicated adjustment measures have to be undertaken in order to cause the individual fuel sprays to collide within a narrowly limited region.
In addition, the slot nozzle opening can itself have a conical configuration so that the fuel spray forming converges, even without the provision of various deflection elements, in a narrowly limited volumetric region and there leads to the desired collision events.
The arrangements described above for generating extremely small fuel droplets are particularly suitable for use in double-cone burners, of which a preferred example is given in EP 0 321 809 B1.
This type of burner is considered to be a successful initial type for burners which are designed for firing using liquid fuels. In it, the liquid fuel is introduced by means of a nozzle arrangement attached centrally to the hollow conical space and is introduced to the inside of the combustion chamber in the form of a conically forming fuel spray. The conical fuel spray is surrounded by a rotating combustion air flow which enters a hollow conical space tangentially and is stabilized by this means. It is only in the region of the vortex collapse, i.e. in the region of the so-called reverse flow zone, that the optimum, homogeneous fuel concentration is achieved over the cross section, so that the ignition of the fuel spray takes place in this region.
As a supplement to the nozzle arrangement of the burner described above or, indeed, even instead of the nozzle arrangement used in the known burner, the previously described appliances in accordance with the invention can be employed for the atomization of liquid fuel, these appliances being capable of generating extremely small fuel droplets even at the time of the ignition process.